1. Field of the Invention
The present invention relates to an apparatus for a partial oxidation synthesis gas generation of carbon monoxide, carbon dioxide and hydrogen from the combustion of a fossil fuel source in the presence of water and oxygen. Specifically, the present invention relates to a coating for a shielding device utilized in a fuel-injection burner assembly.
2. Background of the Invention
Synthesis gas mixtures comprising carbon monoxide and hydrogen are important commercially as a source of gaseous feed stocks, such as hydrogen, for hydrogenation reactions and as a source of feed gas for the synthesis of hydrocarbons, oxygen-containing organic compounds or ammonia.
Generally, in a synthesis gas operation a fuel stream composed primarily of a pumpable slurry of finely particulated coal and water are sprayed along with an oxidizer into the refractory-lined combustion chamber of the synthesis gas generator. The oxidizer gas contains substantial quantities of free oxygen to support the combustion reaction of the coal. The combustion reaction components of fuel and oxidizer are sprayed under significant pressure, typically about 80 bar, into the synthesis gas combustion chamber. A hot gas stream is produced in the combustion chamber at a temperature in the range of about 700.degree. C. to about 2500.degree. C. and at a pressure in the range of about 1 to about 300 atmospheres and more particularly, about 10 to about 100 atmospheres. The effluent raw gas stream from the gas generator includes such gasses as hydrogen, carbon monoxide, carbon dioxide and can include other gases such as methane, hydrogen sulfide and nitrogen depending on the fuel source and reaction conditions.
The partial combustion of a sulfur bearing hydrocarbon fuel such as coal with oxygen-enriched air or with relatively pure oxygen to produce carbon monoxide, carbon dioxide and hydrogen presents unique problems not encountered normally in the burner art. It is necessary, for example, to effect very rapid and complete mixing of the reactants, as well as to take special precautions to protect the burner or mixer from over heating. Typically, the fuel injection nozzle serving the combustion chamber is configured to have the slurry fuel stream concentrically surround a first oxidizer gas stream along the axial core of the nozzle. A second oxidizer gas stream surrounds the fuel stream annulus as a larger, substantially concentric annulus. Radially surrounding an outer wall of the outer oxidizer gas channel is an annular cooling water jacket terminated with a substantially flat end-face heat sink aligned in a plane substantially perpendicular to the nozzle discharge axis. Cool water is conducted from outside the combustion chamber into direct contact with the backside of the heat sink end-face for conductive heat extraction.
Because of the reactivity of oxygen and sulfur contaminants with the burner metal, it is imperative to prevent the burner elements from reaching those temperatures at which rapid oxidation and corrosion takes place. In this respect, it is essential that the reaction between the hydrocarbon and oxygen take place entirely outside the burner proper and prevent localized concentration of combustible mixtures at or near the surfaces of the burner elements. Even though the reaction takes place beyond the point of discharge from the burner, the burner elements are subjected to heating by radiation from the combustion zone and by turbulent recirculation of the burning gases.
Moreover, it is believed that a confluence of a recirculated gas flow stream with the nozzle emission stream generates a standing eddy of hot, turbulent combustion product comprising highly corrosive sulfur compounds. These hot, corrosive compounds surround the nozzle discharge orifice in a turbulent manner and scrubs the heat shield face at the confluence.
For these and other reasons, prior art burners are characterized by failures due to metal corrosion about the burner tips, even when these elements have been water cooled and where the reactants have been premixed and ejected from the burner at rates of flow in excess of the rate of flame propagation.
Efforts to ameliorate these harmful effects on the injector nozzle have been disclosed. For example, U.S. Pat. No. 5,934,206 discloses a heat shield having a plurality of ceramic tiles, each covering the end face of a respective arc segment of the annulus around the nozzle. The tiles are formed of a refractory ceramic or other high melting point material as individual elements. The individual tiles are secured to the coolant jacket end face by a high temperature brazing compound.
U.S. Pat. No. 5,954,491 discloses a ceramic heat shield that is mechanically secured over the water jacket end-face of the injector nozzle. This heat shield is formed as an integral ring or annulus around the nozzle orifice. The outer face of the heat shield is substantially smooth and uninterrupted to provide minimum contact with the reaction gases and reduced opportunity for reactive combination. The inner face of the heat shield, i.e., that side contiguous with the water jacket end-face, includes a plurality of socket pairs, each pair in radial alignment around the heat shield annulus. A bayonet channel extends from the outer perimeter of the heat shield, between and parallel with the outer and inner heat shield faces, and through each socket pair. A corresponding number of mounting studs project from the water jacket end-face. The studs are appropriately positioned to be in register with the sockets. Each stud includes an aperture that aligns axially with respective bayonet channel bores. With the heat shield in position against the water jacket end-face and the end-face studs penetrating the heat shield sockets, bayonet wires are inserted along the radial channel bore to deadbolt the heat shield to the water jacket-end face at multiple attachment points.
U.S. Pat. No. 5,947,716 discloses a heat shield having a pair of rings where each ring is a full annulus about the nozzle axis that faces or shields only a radial portion of the entire water jacket face annulus. An inner ring is mechanically secured to the metallic nozzle structure by meshing segments about the nozzle axis. The external elements of these segments (lugs) are integral projections from the external cone surface of the nozzle lip. Each of three lugs projecting from the external cone lip is an arcuate portion of an independent ring fin. The internal perimeter of the inner heat shield ring is formed with a channel having a corresponding number of cuts in the wall to receive and pass the respective external lug elements. When assembled, the inner heat shielding ring is secured against rotation by a spot welded rod of metal that is applied to the nozzle cooling jacket face within a notch in the outer perimeter of the inner ring. Additionally, the outer perimeter of the inner heat shield ring is formed with an approximately half thickness step ledge or lap that overlaps a corresponding step ledge or lap on the internal perimeter of an outer heat shield ring. The outer heat shield ring is secured to the water jacket face by a second set of external lug elements projecting from the outer perimeter of the water jacket face. A cuff bracket around the perimeter of the outer heat shield ring provides a structural channel for receiving the outer set of water jacket lugs. The outer heat shield ring is also held in place by a tack-welded rod or bar.
U.S. Pat. No. 5,273,212 discloses a shielded burner clad with individual ceramic platelets which are arranged adjacent to each other in a mosaic surface-covering manner.
U.S. Pat. No. 5,941,459 discloses an annular refractory insert is interlocked with the fuel injector nozzle at the downstream end proximate the nozzle outlet. A recess formed in the downstream end of the fuel injector nozzle accommodates the annular refractory insert.
A problem with the aforementioned shielding devices recently discovered is that in the case where the shielding device is constructed of a high temperature metal, such as molybdenum, the shielding device is subjected to abnormally high rates of oxidative degeneration associated with the period when the synthesis gas generation chamber is being brought up to temperature. Typically, the burner assembly is subjected to relatively high concentrations of oxygen at temperatures greater than about 600.degree. C. This oxidative degeneration of the metal can lead to failure of the shielding device which results in premature failure of the burner assembly.
Accordingly, there is a need for a heat shielded burner for synthesis gas generation which is an improvement over the shortcomings of prior art appliances, is simple in construction and economical in operation.